Computer storage devices, such as disk drives, use read/write heads to store and retrieve data. A write head stores data by utilizing magnetic flux to set the magnetic moment of a particular area on a magnetic media. The state of the magnetic moment is later read by a read head which senses the magnetic fields.
Conventional thin film read heads employ magnetoresistive material, generally formed in a layered structure of magnetoresistive and non-magnetoresistive materials, to detect the magnetic moments of the data bits on the media. A sensing current is passed through the magnetoresistive material to detect changes in the resistance of the material induced by the data bits as they pass the read head.
One conventional type of sensor is a current-in-plane or CIP device as shown in FIG. 1. As can be seen, this sensor 5 has a junction 10, hard bias 40 and leads 50. The junction 10 is a stack of film layers which includes, from the bottom, an antiferromagnet layer 12, a pinned layer 14, a copper spacer layer 16 and at the top of the stack a free layer 18. The junction 10 has sloping sides 11. Typically, the pinned layer 14 is a ferromagnetic layer which, as the name implies, has its magnetization pinned by the antiferromagnetic layer 12. The free layer 18, in contrast is a ferromagnetic layer which has its magnetization set perpendicular to the pinned layer 14, and which is free to change its magnetic orientation in response to a magnetic fields of passing magnetized bits located on an adjacent recording media (not shown).
The hard bias 40 is positioned on both sides of the junction 10. The hard bias 40 includes an underlayer 42, which can be chromium (Cr), and a permanent magnet layer 46, such as cobalt chromium platinum (CoCrPt). The underlayer 42 is laid directly over each side 11 of the junction 10, and the permanent magnet layer 46 is positioned over the underlayer 42. Both the underlayer 42 and the permanent magnet layer 46 overhang and contact the upper surface 20 of the free layer 18. The underlayer 42 contacts the upper surface 20 at end 44 and permanent magnet layer 46 contacts the upper surface 20 at end 48.
Biasing is critical to the proper operation of the sensor 5. The hard bias 40 acts to stabilize the response of the sensor 5 and sets the quiescent state of the sensor. That is, the hard bias 40 stabilizes the domain structure of the free layer 18 to reduce noise. In CIP sensors, such as anisotropic magnetoresistive and spin valve devices, the hard bias 40 functions to set the magnetization of the free layer 18 in a longitudinal direction by pinning the magnetization at each end 22 of the free layer 18. This prevents formation of closure domains at the ends 22. Without this pinning, movement of the end domains can cause hysteresis in the magnetoresistive response of the device. Typically, in CIP devices the hard bias 40 is formed adjacent to and partially overlying the edges 22 of the free layer 18.
As can be seen in FIG. 1, on top of each permanent magnet layer 46 is a lead 50. The lead 50 is made of a conductive material, such as, gold, silver or copper. The lead 50 is laid on both sides of the sensor 5. The lead 50 has ends 52 which each contact the upper surface 20 of the free layer 18 and at or about the edges 22 of the free layer 18. In this manner, the leads 50 can provide an electrical current to and across the junction 10.
Flowing a current through the sensor allows changes in the magnetization of the adjacent magnetic media to be detected as changes in the electrical resistance of the sensor 5. This is because the free layer 18 is free to change its magnetic orientation in response to passing magnetized bits on the recording media. In other words, the magnetized bits on the recording media cause a change in the relative magnetization between the pinned layer 14 and the free layer 18. The change in magnetization causes the electrical resistance of the layer to change as well. Therefore, data can be read by measuring changes in the current passed through the sensor 5 as the recording media is passed by the sensor 5.
An improved type of sensor is the current-perpendicular-to-the-plane or CPP sensor. In a CPP sensor, such as a multilayer giant magnetoresistive (GMR) device or a spin dependent tunneling (SDT) device, the quiescent state of the device has antiparallel magnetic alignment of the magnetoresistive element layers for maximum resistance. In a CPP sensor, the current flows perpendicular to the planes of the layers of the sensor and not parallel as is the case with a CIP sensor. The increase in magnetoresistance (MR) values associated with CPP devices make the CPP sensors more sensitive and therefore allow for the use of smaller data bits, which increases the overall data storage of the disk.
Although the layering of the junction of a CPP sensor is similar to a CIP sensor, the positioning of the leads is completely different. Instead of positioning leads on each side of the device, CPP devices use a top lead positioned above the free layer and a bottom lead positioned below the antiferromagnet layer. Current flowing between the leads passes in a perpendicular manner through the layers of the CPP sensor.
Unfortunately, because of the perpendicular current flow of CPP devices, and because hard bias materials are electrically conductive, CPP devices cannot have the hard bias contacting the sides of the layers of the film stack as is the convention with CIP devices. If the hard bias is laid over the sides of the stack, the hard bias will cause electrical shorting between layers of the film stack to occur. Such shorting will dramatically reduce the performance of the CPP device or render it completely useless.
Thus, a CPP device is sought which is hard biased in a manner which will not cause shorting. Likewise, to produce such a hard biased CPP device, a method of fabrication is sought. The device must prohibit shorting and yet provide sufficient bias to properly pin the magnetization at each end of the free layer, so as to prevent formation of closure domains at the ends of the free layer and hysteresis in the magnetoresistive response of the device. The method must provide the fabrication of such a device in a manner which minimizes the cost and time of manufacture.
The apparatus of the present invention is embodied in a magnetic field sensor having a magnetoresistive element, a magnetic bias layer for biasing the magnetoresistive element with a magnetic field, and an electrical insulator positioned between the magnetic bias layer and the magnetoresistive element. The insulator prevents the flow of electrical current between the magnetoresistive element and the magnetic bias layer and at least a portion of the insulator allows passage of the magnetic field from the magnetic bias layer to the magnetoresistive element.
In at least one embodiment, the electrical insulator has a lower insulator and an upper insulator which are in direct contact with one another, such that the magnetic bias layer is isolated from the magnetoresistive element. The upper and lower insulator are made of either Al2O3, SiO2, Ta2O5 or Si3N4. The lower insulator has a thickness between 50 xc3x85 and 300 xc3x85 and the upper insulator a thickness between 300 xc3x85 and 1000 xc3x85. The lower insulator is positioned between the magnetoresistive element and the magnetic bias layer and overlays at least a portion of the magnetoresistive element.
The magnetic bias layer overlays the lower insulator and the upper insulator overlays the magnetic bias layer. The magnetoresistive element has a top surface. The magnetic bias layer can have a tapered end. At least a portion of the tapered end overhangs the top surface of the magnetoresistive element. The magnetic bias layer has an underlayer and a magnetic layer which is positioned over the underlayer. The underlayer has a thickness between 50 xc3x85-100 xc3x85 and can be made of either chromium or nickel aluminum. The magnetic layer has a thickness between 500 xc3x85-2000 xc3x85 and can be made of either cobalt chromium, cobalt chromium platinum, cobalt chromium platinum tantalum, cobalt chromium tantalum or cobalt platinum. The magnetic bias layer has a MrT (the product of remanent magnetization and thickness) substantially equal to about 3 to 12 times the MrT of the magnetoresistive element.
The method of the present invention is embodied in a method for fabricating a magnetic field sensor having the steps of forming a magnetoresistive element, forming a lower insulator with a main section and an end section, over at least a portion of the magnetoresistive element, forming a magnetic bias layer over the main section of the lower insulator, and forming an upper insulator over the magnetic bias layer and over the end section of the lower insulator, such that the magnetic bias layer is electrically insulated from the magnetoresistive element.
In at least one embodiment of the method, when the magnetic bias layer is formed, it is shaped to have a tapered end, a portion of which can overhang the magnetoresistive element. The steps of forming the upper and lower insulators can be performed by deposition methods including ion beam sputtering, rf sputtering, reactive sputtering and chemical vapor deposition. The step of forming the underlayer of the magnetic bias layer can be performed by either ion beam deposition, rf sputtering, DC magnetron sputtering or electron beam evaporation. Similarly, the step of forming the magnetic layer can be performed by either ion beam deposition or DC magnetron sputtering.
In one embodiment of the method, the steps include depositing a film stack on a bottom lead, patterning the film stack and bottom lead, defining a magnetoresistive element with a top sensing layer from the film stack, depositing a lower insulator with a main section and an end section over a portion of the magnetoresistive element at least adjacent to the top sensing layer, depositing a magnetic bias layer over the main section of the lower insulator, depositing an upper insulator over the magnetic bias layer and over the end section of the lower insulator, and depositing a top lead over the magnetoresistive element such that the top lead is in contact with the lead portion of the top sensing layer. Where the magnetic bias layer which is deposited has a magnetic field sufficiently strong to magnetically bias the top sensing layer of the magnetoresistive element.